Chemical transformations of bile salts by the intestinal microflora

Bile acids are synthesized as primary bile acids in the liver from cholesterol. In humans and all other mammals, bile acid synthesis is the predominant pathway for elimination of cholesterol. After conjugation with glycine or taurine the bile acids are secreted in the bile and via the bile into the intestinal tract where they assist in the digestion and absorption of fat. The largest part of the bile acids are reabsorbed and via the portal blood return to the liver. A small fraction escapes from the enterohepatic circulation and is excreted in the faeces. In the intestinal tract, microorganisms that are part of the intestinal microflora can transform the primary bile acids into secondary bile acids. These secondary bile acids can also be reabsorbed and return to the liver where they can again be transformed into the so-called tertiary bile acids before they are again excreted into the bile. In this chapter the transformations of bile acids by the intestinal microflora and the impact of these bile acid transformations on the host will be discussed.